1. Field of the Invention
The present invention relates to an intake air amount detecting apparatus for use with internal combustion engines of the type having an electronically controlled fuel metering system.
2. Description of the Prior Art
In the past, it has been confirmed that maintaining the air-fuel ratio of the mixture supplied to the engine at a certain ratio provides a very powerful means of purifying the exhaust gases discharged from the engine, and systems for detecting the amount of air drawn into the engine have been used in which the volume flow rate of intake air is indirectly detected in accordance with the engine rotational speed and the intake manifold vacuum or in accordance with the engine rotational speed and the throttle valve opening. The system of this type is disadvantageous in that since the amount of air drawn through the intake pipe is measured indirectly, the measurement error will be increased due to the effects of the changes in characteristics among engines of the same type caused during the manufacturing process, deterioration of the engine, unsatisfactorily adjusted clearance of the intake and exhaust valves of the engine, changes in performance of the air cleaner with time, etc., and moreover due to the fact that the intake air amount is measured in terms of volume flow rate, it is necessary to provide absolute pressure compensation thus making the system more complicate and expensive.
To overcome these deficiencies, a system has been proposed which includes a branch pipe disposed in the intake manifold of an engine and having two passages and first and second temperature responsive resistors disposed in the passages, and an electric heater disposed upstream of the temperature responsive resistor in one of the passages, whereby the voltage applied to the electric heater is controlled to maintain the temperature difference between the first and second temperature responsive resistors at a predetermined value and to thereby detect the amount of intake air in accordance with the applied voltage. One such system is described in U.S. Pat. No. 3,975,951, issued Aug. 24, 1976, to Kohama et al, and assigned to the same assignee of the present application. With this construction, however, the applied voltage has a complicate functional characteristic with respect to the intake air amount, and consequently if this applied voltage is utilized as a signal indicative of the intake air amount, it is necessary to apply a complicate correction to the calculation in accordance with the voltages of the resistors or the like.